Level Skipping

Some levels in Castle Crashers can be bypassed while a player is in a cooperative game with another player who already has beaten that level.

A player may notice that, after exiting a game and reloading it, some levels will remain locked, even after completion. This is noticeable as early as the Forest Entrance level. To unlock this level permanently, Thieves' Forest must be completed. If playing with someone who has already completed Thieves' Forest, the player only needs to complete this level to unlock Forest Entrance, demonstrating that some levels can be skipped, as long as the one following it is completed. This means players can complete the game for a character, Skull, or for Insane Mode relatively quickly.

List of Mandatory Levels

If playing with someone who's completed every level, the following levels are required:

Cyclops' Fortress (You may exit to map after the Boss Chest is opened or, if accumulating gold as you go by, you may exit to map after all gold is collected)

Lava World (Depending on the version you're playing on, you may exit to map as soon as the cutscene of the Necromancer resurrecting the Cyclops starts and/or in some versions, you may need to stay for the entire cutscene)

Ice Castle (You may exit to map the moment you see the death animation of the Frost King starting; not recommended for if you're accumulating gold as you progress; if this is the case, you may exit to map after all gold is collected)

Notes

You must quit to the Main Menu, start the game again, and defeat the final boss a second time or else your skull and/or character will not be unlocked. The first time you beat him unlocks the level, the second time the skull and/or character.

If you skip any level on this list then you will not unlock your respective skull and/or character.

Not every character unlocks another character when you beat the game with them. Because of this, it's recommended that you beat the game with characters that unlock another character first, then move to the characters with no unlocks.

Trivia

By using this technique, you'll only play 16 of the 36 levels. That's only 44% of the levels, meaning that 56% of the game is just filler.